(a) Field of the Invention
The present invention relates to an organic light emitting device and manufacturing method therefore.
(b) Description of the Related Art
As display devices have been getting larger, a flat panel display device that takes up little space is increasingly required. An organic light emitting device, one of the flat panel display devices, is being rapidly developed.
The organic light emitting device has an organic thin film sandwiched between two opposing electrodes and is a multi-layered structure using different materials in order to enhance efficiency and stability. An organic light-emitting device includes an anode, a hole injection layer into which holes are injected from the anode, a hole transport layer for transporting holes, an emission layer for combining holes with electrons, an electron transport layer for transporting electrons, and a cathode. If holes and electrons generated from the anode having a high work function and the cathode having a low work function, respectively, are injected into the emission layer through the hole injection layer/hole transport layer and the electron injection layer/electron transport layer, excitons are generated within the emission layer. When the excitons are extinguished, light corresponding to energy thereof is generated and emitted.
In the development of a high-efficiency organic light emitting device, a technology for injecting electrons from the cathode and for injecting holes from the anode to the emitting layer without generation of an energy barrier is of importance.
Magnesium (Mg), which has a low work function of 3.6 electron volts (eV), is used to reduce the energy barrier that is an issue when electrons are injected from a metal electrode to an organic compound that is known as an electrical insulator. Since Mg is easily oxidized, is unstable and has poor surface cohesion to organic materials, silver (Ag), which is relatively stable and has a high work function and good cohesion to a surface of organic materials, is alloyed with Mg (hereinafter, referred to as “Mg—Ag”) for use (Tang et al., Appl. Phys. Lett. 51, 913 1987).
Further, a research group of Toppan Printing Company (51st periodical meeting, The Japan Society of Applied Physics, Preprint 28a-PB-4, p.1040) discovered that, if lithium (Li) (work function of 2.9 eV), being an alkali metal and having a lower work function than Mg, is alloyed with aluminum (Al) (work function of 4.2 eV) to form a stable electron injection cathode, a lower driving voltage and a higher light emitting brightness than that of an organic light emitting device using a Mg alloy can be obtained.
In order to further improve device efficiency and lifetime, the electrode layer can be formed of a mixture material or an additional functional layer can be introduced. Until now, two steps of depositing a Mg—Ag alloy thin film having a Mg/Ag weight percentage of 7:3 (ratio of 10 to 1 in terms of atomic ratio) and then depositing a Ag thin film have generally been performed, or an Al thin film has been used as a cathode and a LiF buffer layer has been deposited to be used together.
However, these methods have a drawback in that, as a plurality of thin films are used, a phenomenon of electron trapping occurring between the thin films or occurrence of interface roughness between heterogeneous materials causes electron mobility to be reduced and an electron injection effect to be deteriorated.